Since the early 1960's, quartz crystals have been used to monitor thin film coating processes used in the fabrication of optical devices such as lenses, filters, reflectors and beam splitters. Although initially employed as an aid to optical monitors to provide information on the rate at which the film is deposited, quartz crystal sensors became relied upon to indicate and control optical layer thickness in automated deposition systems.
Research in fields such as nanotechnology, biosensors, thin film displays, and high-speed optical communications have increased the complexity of thin film structures. While an antireflection coating consisting of a single layer of magnesium fluoride may have been sufficient 20 years ago, current designs may call for a 24-layer stack of alternating refractive index films. With high-speed optical communications, this stack can be increased ten-fold or more, leading to filters comprised of up to 256 or more layers.
Quartz sensor instruments have been used to measure film thickness by monitoring a change in the frequency of vibration of a test crystal coated simultaneously with process substrates. Quartz is a well-known piezoelectric material, i.e., if a bar of quartz is bent, it will develop a voltage on opposite faces. Conversely, if a voltage is applied, the bar will bend. By applying alternating voltage to such a bar, the bar will vibrate or oscillate in phase with the voltage.
At a specific frequency of oscillation, quartz will vibrate with minimal resistance, much like a tuning fork rings when struck. This natural resonance frequency is used as the basis for measuring film thickness. By adding coatings to the crystal surface, the resonance frequency decreases linearly. If the coatings are removed, the resonance frequency increases.
In a quartz crystal thickness monitor, the quartz crystal is coupled to an electrical circuit that causes the crystal to vibrate at its natural (or resonant) frequency, which for most commercial instruments is between 5 and 6 MHz. A microprocessor-based control unit monitors and displays this frequency, or derived quantities, continuously. As material coats the crystal during deposition, the resonant frequency decreases in a predictable fashion, relative to the rate material arrives at the crystal, and the material density. The frequency change can be calculated several times per second, converted in the microprocessor to Angstroms per second and displayed as deposition rate. The accumulated coating can be displayed as total thickness.
The sensitivities of these sensors are remarkable. A uniform coating of as little as 10 Angstroms of aluminum will typically cause a frequency change of 20 Hz, easily measured by today's electronics. As the density of the film increases, the frequency shift per Angstrom increases.
Typically, quartz crystal sensors used in thin film vacuum deposition systems, coated with opaque electrodes, are placed into a metal housing, often water cooled, with one side open to the coating material source. As the coating material source turns on, material coats the open face of the crystal, as well as any substrates or objects situated nearby. This is the basis for process monitoring and control with a vibrating quartz crystal sensor, or quartz crystal microbalance (QCM) as it is often called. A significant limitation of QCM's is that they reveal no information about the optical properties of the deposited film, only the physical (thickness or mass/density).
A representative example of a conventional chip 80 for use in a thin film monitor is depicted in FIGS. 8–10. FIG. 8 is a top view showing a first gold electrode 81 and a piezoelectric element 83. FIG. 9 is a bottom view showing a second gold electrode 82. FIG. 10 is a sectional view taken along line 10—10 in FIG. 8. The region in which the most pronounced vibration will occur (upon passing a current between the first and second electrodes 81 and 82) is in the center of the chip.
In the process of manufacturing elements, e.g., optical elements, such as lenses, glass filters, and telecommunication components, a second thin film coating measurement technique, known as Optical Monitoring (OM) has also been employed. In this set-up, a glass disc, or monitoring chip, is placed near the objects to be coated. A beam of light is introduced into the process chamber, either by a lamp or laser shining through a window into the chamber from the outside, or a suitable light source inside the chamber. On the other side of the chip (in or out of the chamber), there is positioned a light detection device, such as a photocell or photo multiplier tube. As the coating covers the monitor chip, the growing film or coating manifests the optical properties of interference, reflection or absorption. This changes the amount of light reaching the detection device. During the coating process, these changes are continually monitored and can be used to control the coating process. A limitation of optical monitoring is that it is practically impossible to detect small (10 Angstrom) level thickness changes, especially in real-time fashion to the extent that the coating source can be accurately controlled. This is critical, since many optical films are dependent on the rate of the coating in order to insure proper refractive index, density, stress and other film properties.
U.S. Pat. No. 4,311,725 discloses an apparatus for sensing and controlling the deposition of a thin film on to a substrate 12 from a gas or vapor phase, in which the optical reflectance or transmittance is sensed, and in which the resonant frequency of a crystal 14 also exposed to the deposition is sensed. The patent discloses a method of sensing and controlling the deposition of a thin film onto a substrate from a gas or vapor phase, the method comprising: sensing the optical reflectance or optical transmittance or electrical resistivity of the deposited film and providing a first signal having a value representative of the value of the sensed property; arranging an oscillatable crystal adjacent to the substrate with at least one surface of the crystal exposed to the gas or vapor phase; determining by timing and calculating means the absolute resonance frequency of the oscillatable crystal and deriving a second signal having a value representative of the resonance frequency; determining over a first predetermined interval of time the quotient of the change in the first signal and the change in the second signal; providing an output signal dependent on the quotient; and controlling the variable of the deposition process in accordance with the output signal.
U.S. Pat. No. 6,616,818 discloses an apparatus and method for coating substrates. According to the patent, it is preferred that the apparatus include a detection means adapted to detect certain conditions within the chamber, and that the detection means include a vapor detection means adapted to detect vaporized source material within the chamber. According to the patent, preferably the vapor detection means comprises at least one detector capable of detecting the vapor. The patent discloses that it is preferable to use more than one detector, and that in practice, at least three vapor detectors are preferred, and, ideally, there should be a minimum of six detectors. According to the patent, the detectors are most preferably distributed quartz crystals, and the crystals change the frequency of oscillation as the amount of vaporized material builds up, and hence provide a measure of the amount of vaporized material.
U.S. Pat. No. 6,616,818 states that it is preferred that the detection means additionally, or instead, comprise an optical monitoring detection means adapted to detect the level of deposition of the source material on the substrate, the substrate detection means including at least one detector capable of detecting the change in light level due to optical interference from the film surfaces as the thickness of the deposit on the substrate changes. In practice, the patent discloses, a monochromatic beam at the desired design wavelength λ0, is obtained in the monitoring system by incorporating a monochromator, and a measure of the transmittance of the deposited coating is obtained in terms of the detector output. Specific transmittance is a function of the film thickness. The patent states that it is preferable to use more than one detector, spatially distributed, as this, according to the patent, provides a measure of the level of deposition over the whole of the substrate. In practice, the patent states, at least three substrate detectors are preferred at center, middle and edge substrate positions. The substrate detection means may comprise a light source and a light detector each arranged on opposite sides of the substrate.
U.S. Pat. No. 6,616,818 discloses an apparatus 10 as depicted in FIG. 1. The apparatus 10 comprises a vacuum chamber 12, a rotatable substrate carrier 14 positioned within the vacuum chamber 12 which carries one or more substrates 16 to be coated (or cleaned). A rotary drive mechanism 18 is provided to rotate the substrate carrier 14. Also within the vacuum chamber there is disposed a vaporization means generally designated 20. The apparatus 10 further includes a detection means in the form of a plurality of distributed quartz crystals 56 which are disposed around the chamber 12. The detection means of the apparatus 10 further includes an optical detection means in the form of laser light source 62 and a light detector 64. The light source is provided is connected to three fiber optic cables 66, which deliver three light beams to the chamber 12. Three optical monitor witness pieces 68 are provided on the substrate carrier 14 to enable the light to pass through the substrate carrier 14. The base of the chamber 12 is provided with three fiber optic cables 70, each of which is aligned with a respective one of the fiber optic cables 66, to carry the light to the detector 64.
A paradigm shift is underway in quartz crystal process monitoring. In many applications, crystals become the keys to success. No matter how significant a breakthrough may be in optics, be it materials, geometry, process design or application, if a coating of any sophistication is required, the weak link is how accurately that coating can be measured. As technology closes in on manipulating Angstrom-level properties of matter, the need for reliable thin film metrology rises to a new level of importance. There is, accordingly, an ongoing need for improvements in measurement and process monitoring of translucent coatings, e.g., coatings deposited by vacuum or atmospheric deposition techniques. The expression “translucent,” as used herein, means that at least some portion of light directed toward a first side of the structure passes into and exits the structure.